By Zoe Catchpole, Political Reporter and Philip Whiteside, News Reporter

Northern Ireland's First Minister Arlene Foster has hit out at Deputy First Minister Martin McGuinness after he resigned over a scandal she has been involved in.

She said his actions were "not principled" and "purely political" after he quit in protest at a botched energy scheme, bringing about the end of the historic pact between the Democratic Unionists and Sinn Fein.

The Deputy First Minister said there would be "no return to the status quo", prompting a collapse of the fragile Northern Irish government established when Rev Ian Paisley agreed to share power 10 years ago.

Sinn Fein's Mr McGuinness has been calling on Mrs Foster to resign over her handling of energy scheme, known as "cash for ash", that could have cost the taxpayer £400m.

Video:McGuinness resigns, says DUP is in 'fool's paradise'

But she has refused to go, prompting Mr McGuinness to quit.

He said: "Over the last 10 years I have worked with DUP leaders and reached out to unionists on the basis of equality, respect and reconciliation.

"Sinn Fein will not tolerate the arrogance of Arlene Foster and the DUP."

Mrs Foster said: "I am disappointed that Martin McGuinness has chosen to take the position he has today.

Image:Martin McGuinness and Arlene Foster have been sharing power for just over a year

"His actions have meant that, at precisely the time we need our government to be active, we will have no government and no way to resolve the RHI (Renewable Heat Incentive) problems.

"Let me make it clear the DUP will always defend unionism and stand up for what is best for Northern Ireland and it appears from the Deputy First Minister's resignation letter that is what annoys Sinn Fein the most."

Northern Ireland Secretary James Brokenshire said that unless Sinn Fein nominates a replacement in the next seven days, he would have no choice but to call an Assembly election.

Image:Theresa May, (C), with Northern Ireland First Minister Arlene Foster and Deputy First Minister Martin McGuinness in July

He said: "I would urge Northern Ireland's political leaders to take the necessary steps to work together to find a way forward and I will work with all parties and the Irish government to this end."

The state-funded environmental scheme was supposed to offer businesses a proportion of the cost of running eco-friendly boilers.

Video:Arlene Foster: Resignation calls 'misogynistic'

But the subsidy tariffs were set too high and, without a cap, resulting in Stormont paying out significantly more than the price of fuel.

In one case, a farmer was allegedly expected to pocket around £1m in the next 20 years for heating an empty shed.

Speaking last week, Mrs Foster said she was ready for an election if Sinn Fein pulled out.

Before Christmas, she told Stormont the lack of cost controls on the scheme, which meant businesses were receiving more in subsidies than they were paying for renewable fuel, had been her "biggest political regret".

He said: "... the power-sharing government of Northern Ireland stands on the basis of an agreement to share power between Sinn Fein and the Democratic Unionists so when one leg of the power-sharing government disappears the whole thing collapses."